Due to the increasing degree of integration of electrical circuits and because of the errors occurring during production and in service, the expenditure for testing and diagnosing such circuits is also increasing. Complex circuits are frequently diagnosed and tested by using scan paths. In this context, the outputs of the electrical circuits also form the outputs of the scan paths. In circuits which have many outputs, there is usually a correspondingly large amount of output data which must be processed in the test. This takes a long time and requires a large amount of storage space.
In DE 103 38 922, test or diagnostic values with a word width N are input into a circuit to be tested or to be diagnosed. The circuit to be tested or to be diagnosed outputs a test response with a word width M which is compacted into test output data of word width m by a compactor, where m is smaller than M. Compactors use special linear automatons such as multi-input shift registers (MISHR) or multi-input linear feedback shift registers (MILFSR).
Within digital circuits, there are voltage nodes, the voltage levels of which are unknown. These voltage levels are called unknown values or abbreviated X values. Unknown values are produced by, for example, non-initialized registers or lines driven at high impedance. As a rule, they do not lead to a malfunction of the electrical circuit.
When output data influenced by these unknown values are compacted, however, the problem arises that these output data cannot be used for assessing the circuit. This is solved by the abovementioned DE 103 38 922 in that a multiplexer in the compactor is adjusted in such a manner that unknown values are replaced by known values. However, this increases the control and calculation effort for the compactor.
The reference “Diagnosis of Scan-Chains by Use of Configurable Signature Registers and Error Correcting Codes”, Proceedings DATE 2004, pp. 1302-1307 by A. Leininger, M Goeseel and P. Muhmenthaler, describes how control signals can be used for a compactor in different successive runs so that in each case the syndrome values of an error-correcting code are output at the output of the compactor. By this means, faulty scan cells can be located.
U.S. Pat. Nos. 6,510,398, 5,930,270 and 20040230884 and WO 01/38889 show other test methods with compactors.